A textured glass, glass-ceramic, or ceramic surface, particularly for use as a magnetic memory disk substrate, and method of texturing.
For certain applications, it is necessary to produce a roughened, or textured, surface on a glass, glass-ceramic, or ceramic surface. An application of current interest is the substrate for a magnetic memory disk.
A magnetic memory storage device consists essentially of a head pad, or slider, and a rigid information disk. The head pad supports an element capable of magnetically reading or writing data on the information disk. The information disk is composed of a rigid substrate carrying a coating of magnetic media which stores information.
Technical development in the computer field has led to demand for increased disk capacity and higher a real density. In order to thus read and write data, a recording head must fly closer to the surface of the disk. As a consequence, the characteristics of the substrate become even more critical with the trend being to flatter, smoother surfaces.
As the recording head moves closer to the media, it is essential that the coefficient of friction of the substrate/media surface be low. The characteristics that lead to low friction are low bearing area and surface roughness. These are in conflict with surface characteristics that promote low glide, low noise, and high bit densities, i.e., smooth and flat surfaces. As a result, this conflict causes compromises to be made in surface design, i.e., not too smooth to cause frictional problems, but sufficiently smooth to foster low glide.
It is conventional practice to employ an information disk having an aluminum substrate upon which a nickel-phosphorous alloy may be plated. To satisfy technical requirements, it has been proposed to at least partially texture the surface on the substrate. Various texturing means have been proposed including stamping, abrading and etching techniques.
More recently, it has been proposed to produce a textured surface area, on either the metal substrate or alloy coating, by a laser treatment. In particular, it has been proposed to index a Nd:YAG or Nd:YLF laser over the surface to produce a pattern of cavities or pits.
Certain problems have been encountered in using a metal substrate. The thin metal tends to lack rigidity and body strength. It is also prone to surface damage. This has led to a search for an alternative material.
U.S. Pat. No. 4,971,932 (Alpha et al.) describes the fabrication of memory storage devices using a glass-ceramic material as the substrate for the disk. A glass-ceramic containing a silicate as the predominant crystal phase is disclosed for that purpose.
The patent states that its glass-ceramic substrate exhibits very high body strength and fracture toughness; also a much harder surface than aluminum metal. The inherently textured surface of a glass-ceramic was thought to be of special value because no special texturizing operation would be required. The surface texture of the glass-ceramic surface could be tailored to a particular average roughness value through base composition selection, or by grinding and polishing, or by chemical etching.
U.S. Pat. No. 5,534,321 (Alpha et al.) proposes a substrate having a surface divided into two parts: (1) a smooth area exhibiting a Rpv value less than about 10 nm, and (2) a low friction area exhibiting a RPV value greater than about 10 nm, conventionally up to about 50 nm. The application further proposes using as a substrate material a photonucleable glass such as described in U.S. Pat. No. 2,628,160 (Stookey). These glasses are capable of being rendered chemically sculpturable, that is, etchable in a selected area. The procedure involves a combination of exposing the selected area to short-wave radiation and a subsequent heat treatment. A disk substrate of the glass was found to be capable of being treated so that the two required surface textures are produced on one surface. The patent is incorporated by reference for further details.
The procedure described by Alpha et al. was technically effective in producing the desired texture on a substrate. However, like any etching procedure, it was neither simple, nor inexpensive, to practice. Accordingly, a more practical alternative became desirable.
The present invention provides an alternative means of texturing a surface on a glass, a glass-ceramic, or a ceramic body. The method is convenient to practice while permitting close dimensional control of the texture produced. It is generally applicable, but is particularly useful in producing a magnetic head substrate that has either an overall, or a partially, textured surface. Thus, it may be employed to produce a substrate with an annular textured area such as described in the Alpha et al. -321 patent.
The article of the invention is a glass, glass-ceramic, or ceramic body having a textured pattern over at least a portion of its surface, the pattern comprising depressed, conical areas at spaced intervals on the surface.
The invention further resides in a method of texturing at least a portion of a surface on a glass, a glass-ceramic, or a ceramic body, the method comprising exposing the surface to laser radiation having a wavelength that is substantially absorbed by the material of the surface, continuing the exposure for a time, and at an intensity, such that a cavity of desired depth is formed.